A wide variety of substrates require protection from adverse environmental conditions such as moisture, insects, plant life, and corrosion deposits. For some substrates such as electrical components, it is desirable that a protective cover or material be provided such that it is possible to easily reenter and work on the substrate, for the purpose of resplicing, repairing, etc.
It is known to protect an electrical contact with grease, the grease being contained in a container which is applied to the electrical contact. See, for example, Farrar, Jr., U.S. Pat. No. 3,897,129 (1975). However, grease is thixotropic, limiting the ways in which it can be used. Furthermore, when reentry is attempted, the grease remains on the contact when the container is removed and must be cleaned off before electrical work can begin.
Electrical contacts can also be potted or encapsulated in a container with a two-part liquid composition prepared by mixing ingredients which will slowly react together. Before the ingredients have reacted and cured, the mixture is poured into the container, where it cures around the contacts into an encapsulating composition. See, for example, Arnold, U.S. Pat. No. 4,375,521 (1983) and Groves et al., U.S. Pat. No. 4,102,716 (1978). However, this procedure entails preparation of the liquid composition at the work site, waiting while the composition cures, and the provision of a container around the contacts, into which the composition can be poured and allowed to cure. Furthermore, when reentry is required, the cured composition cannot be easily removed.
Another method of protecting substrates employs a gel which has been preformed in the absence of the substrate. The gel and the substrate to be encapsulated are pressed against each other (e.g., with aid of a support member or cover), deforming the gel into close and conforming contact with the substrate. Preferably, at least part of the deformation is elastic deformation. The physical properties of the gel will depend upon the substrate to the protected, the environmental conditions to be protected against, and the method by and apparatus with which the gel and substrate are brought into and maintained in contact. Generally, the gel should be relatively soft and have a relatively high elongation. The term "gel" as used herein denotes a substance having the preceding characteristics, making it suitable for use as described in this paragraph.
Numerous compositions have been proposed for use as gels, including polyolefinic block copolymers, organopolysiloxanes, polyurethanes, and polyureas. See, for example Gamarra et al., U.S. Pat. No. 4,716,183 (1987), Gamarra, U.S. Pat. No. 4,942,270 (1990), Dubrow, U.S. Pat. No. 4,777,063 (1988), Dubrow et al., WO 90/10035 (1990), Debbaut, U.S. Pat. No. 4,634,207 (1987), Rinde et al., WO 91/13109 (1991), and Dittmer et al., U.S. Pat. No. 4,852,646 (1989).
It is often desirable to improve the fire resistance of an organopolysiloxane with additives, where applications requiring superior fire resistance are contemplated. Examples of prior art methods for improving organopolysiloxane fire resistance include Delatorre et al., U.S. Pat. No. 4,102,852 (1978); Seino, U.S. Pat. No. 4,695,597 (1987); Modic, U.S. Pat. No. 4,401,491 (1983); Nakamura, U.S. Pat. No. 4,320,044 (1982); Bobear, U.S. Pat. No. 4,288,360 (1981); Modic, U.S. Pat. No. 4,189,545 (1980); Noble, U.S. Pat. No. 4,184,995 (1980); Laur, U.S. Pat. No. 3,996,188 (1976). Also of interest is RXS Schrumpftechnik, DE 3,723,980 (1989), which relates to fire retardation in polyolefinic plastic materials.
For certain electrical insulating/sealing applications, for example in aviation wire and cable, a strongly flame resistant gel is required. Conventional additives are generally ineffective for producing an organopolysiloxane gel capable of meeting stringent flammability standards. We have found that by combining certain additive packages with an organopolysiloxane having specified aromatic content, organopolysiloxane compositions suitable for use as gels and capable of meeting stringent flammability standards can be produced. We have further discovered that certain of the additives unexpectedly improve the mechanical properties of the gels without compromising sealing performance.